Unit Five. Evolution of Animal Life
A profound innovation marks the origin of the body plan characteristic of the most successful of all animal groups, the arthropods, phylum Arthropoda. This innovation was the development ofjointed appendages.
The name arthropod comes from two Greek words, arthros, jointed, and podes, feet. All arthropods (figure 19.17) have jointed appendages. Some are legs, and others may be modified for other uses. To gain some idea of the importance of jointed appendages, imagine yourself without them—no hips, knees, ankles, shoulders, elbows, wrists, or knuckles. Without jointed appendages, you could not walk or grasp an object. Arthropods use jointed appendages as legs and wings for moving, as antennae to sense their environment, and as mouthparts for sucking, ripping, and chewing prey. A scorpion, for example, seizes and tears apart its prey with mouthpart appendages modified as large pincers.
Figure 19.17. Arthropods are a successful group.
About two-thirds of all named species are arthropods. About 80% of all arthropods are insects, and about half of the named species of insects are beetles.
The arthropod body plan has a second great innovation: Arthropods have a rigid external skeleton, or exoskeleton, made of chitin. In any animal, a key function of the skeleton is to provide places for muscle attachment, and in arthropods the muscles attach to the interior surface of the hard chitin shell, which also protects the animal from predators and impedes water loss.
However, while chitin is hard and tough, it is also brittle and cannot support great weight. As a result, the exoskeleton must be much thicker to bear the pull of the muscles in large insects than in small ones, so there is a limit to how big an arthropod body can be. That is why you don’t see beetles as big as birds or crabs the size of a cow—the exoskeleton would be so thick the animal couldn’t move its great weight. Another limitation on size is the fact that in many arthropods, including insects, all parts of the body need to be near a respiratory passage to obtain oxygen. The reason for this is that the respiratory system (see section 24.1), not the circulatory system, carries oxygen to the tissues.
In fact, the great majority of arthropod species consist of small animals—mostly about a millimeter in length— but members of the phylum range in adult size from about 80 micrometers long (some parasitic mites) to 3.6 meters across (a gigantic crab found in the sea off Japan). Some lobsters are nearly a meter in length. The largest living insects are about 33 centimeters long, but the giant dragonflies that lived 300 million years ago had wingspans of as much as 60 centimeters (2 feet)!
Arthropod bodies are segmented like those of annelids, from which they almost certainly evolved. Individual segments often exist only during early development, however, and fuse into functional groups as adults. For example, a caterpillar (a larval stage) has many segments, while a butterfly (and other adult insects) has only three functional body regions—head, thorax, and abdomen—each composed of several fused segments. Some of the segmentation can still be seen in the grasshopper in figure 19.18, especially in the abdomen.
Figure 19.18. Segmentation in insects.
This grasshopper illustrates the body segmentation found in adult insects. The many segments found in most larval stages of insects become fused in the adult, giving rise to three adult body regions: the head, thorax, and abdomen. The appendages—legs, wings, mouthparts, antennae—are jointed.
Arthropods have proven very successful due to the arthropod innovations of jointed appendages and exoskeletons. The Phylum Facts illustration on the facing page walks you through a succinct overview of arthropod characteristics. About two-thirds of all named species on earth are arthropods. Scientists estimate that a quintillion (a billion billion) insects are alive at any one time—200 million insects for each living human!
Arthropods such as spiders, mites, scorpions, and a few others lack jaws, or mandibles, and are called chelicerates. Their mouthparts, known as chelicerae, evolved from the appendages nearest the animal’s anterior end, as in the jumping spider pictured below. The chelicerae are the foremost appendages located on the head.
The chelicerate fossil record goes back as far as that of any multicellular animal, about 630 million years. One of the most ancient groups contains the horseshoe crabs, with five species surviving today. Horseshoe crabs swim on their backs by moving their abdominal plates and walk on their five pairs of legs. The body of a horseshoe crab is covered by a hard shell, which has a long tailpiece (figure 19.19); there are two compound eyes and two simple eyes on the shell. Horseshoe crabs feed at night, primarily on mollusks and annelids.
Figure 19.19. Horseshoe crabs.
These horseshoe crabs, Limulus, are emerging from the sea to mate.
By far the largest of the three classes of chelicerates is the largely terrestrial class Arachnida, with some 57,000 named species, including the spiders (two poisonous spiders found in North America are pictured in figure 19.20), ticks, mites, scorpions, and daddy longlegs. Arachnids have a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs. The chelicerae consist of a stout basal portion and a movable fang often connected to a poison gland. Pedipalps, the next pair of appendages, may resemble legs, but they have one less segment. In scorpions, the pedipalps are large and pinching. Most arachnids are carnivorous, although mites are largely herbivorous. Ticks are blood-feeding ectoparasites of vertebrates, and some ticks may carry diseases, such as Rocky Mountain spotted fever and Lyme disease.
Figure 19.20. Arachnids.
(a) One of the poisonous spiders in the United States and Canada is the black widow spider, Latrodectus mactans. (b) Another of the poisonous spiders in this area is the brown recluse, Loxosceles reclusa. Both species are common throughout temperate and subtropical North America, but they rarely bite humans.
Sea spiders are also chelicerates and are relatively common, especially in coastal waters. More than 1,000 species are in the class.
The remaining arthropods have mandibles, formed by the modification of one of the pairs of anterior appendages, not necessarily the foremost set of appendages. The foremost set of appendages in the bullfrog ant pictured below is the antennae with the mandibles forming from the next set of appendages. These arthropods, called mandibulates, include the crustaceans, insects, centipedes, millipedes, and a few other groups.
Crustaceans. The crustaceans (subphylum Crustacea) are a large, diverse group of primarily aquatic organisms, including some 35,000 species of crabs, shrimps, lobsters, crayfish, water fleas, pillbugs, sowbugs, barnacles, and related groups (figure 19.21). Often incredibly abundant in marine and freshwater habitats, and playing a role of critical importance in virtually all aquatic ecosystems, crustaceans have been called “the insects of the water.” Most crustaceans have two pairs of antennae (the first pair are shorter and often referred to as antennules, as labeled in figure 19.22), three pairs of chewing appendages (one pair being the mandibles), and various numbers of pairs of legs. The nauplius larva stage through which all crustaceans pass provides evidence that all members of this diverse group are descended from a common ancestor. The nauplius hatches with three pairs of appendages and metamorphoses through several stages before reaching maturity. In many groups, this nauplius stage is passed in the egg, and development of the hatchling to the adult form is direct.
Figure 19.21. Crustaceans.
(a) Dark-fingered coral crab. (b) Sowbugs, Porcellio scaber. (c) Barnacles are sessile animals that permanently attach themselves to a hard substrate.
Crustaceans differ from the insects in that the head and thorax are fused together, forming the cephalothorax, and they have legs on their abdomen as well as on their thorax. Many crustaceans have compound eyes. In addition, they have delicate tactile hairs that project from the cuticle all over the body. Larger crustaceans have feathery gills near the bases of their legs. In smaller members of this class, gas exchange takes place directly through the thinner areas of the cuticle or the entire body. Most crustaceans have separate sexes. Many different kinds of specialized copulation occur among the crustaceans, and the members of some orders carry their eggs with them, either singly or in egg pouches, until they hatch.
Crustaceans include marine, freshwater, and terrestrial forms. Crustaceans such as shrimp, lobsters, crabs, and crayfish are called decapods, meaning “ten-footed,” like the lobster in figure 19.22. Pillbugs and sowbugs are terrestrial crustaceans but usually live in moist places. Barnacles are a group of crustaceans that are sessile as adults but have free-swimming larvae. The larvae attach their heads to rocks or other submerged objects and then stir food into their mouths with their feathery legs.
Figure 19.22. Body of a lobster, Homarus americanus.
Some of the specialized terms used to describe crustaceans are indicated. For example, the head and thorax are fused together into a cephalothorax. Appendages called swimmerets occur in lines along the sides of the abdomen and are used in reproduction and also for swimming. Flattened appendages known as uropods form a kind of compound "paddle" at the end of the abdomen. Lobsters may also have a telson, or tail spine.
Millipedes and Centipedes Millipedes and centipedes have bodies that consist of a head region followed by numerous similar segments. Centipedes have one pair of legs on each body segment, and millipedes have two. This difference is apparent if you compare the leg arrangements in the centipede in figure 19.23a and the millipede in figure 19.23b. The centipedes are all carnivorous and feed mainly on insects. The appendages of the first trunk segment are modified into a pair of venomous fangs. In contrast, most millipedes are herbivores, feeding mostly on decaying vegetation. Millipedes live mainly in damp, protected places, such as under leaf litter, in rotting logs, under bark or stones, or in the soil. The first animal to have lived on land was a millipede; its 420-million-year-old fossil was reported in 2004.
Figure 19.23. Centipedes and millipedes.
Centipedes are active predators, whereas millipedes are sedentary herbivores. (a) Centipede, Scolopendra. (b) Millipede, Sigmoria, in North Carolina.
Insects The insects, class Insecta, are by far the largest group of arthropods, whether measured in terms of numbers of species or numbers of individuals; as such, they are the most abundant group of eukaryotes on earth. Most insects are relatively small, ranging from 0.1 millimeters to about 30 centimeters in length. Insects have three body sections:
1. Head. The insect head is very elaborate, with a single pair of antennae and elaborate mouthparts that are well-suited to their diets. For example, the mouthparts in the mosquito in figure 19.24a are modified for piercing the skin; the long proboscis of the butterfly in figure 19.24b can uncoil to reach down into flowers; and the short mouthparts of the housefly in figure 19.24c are modified for sopping up liquids. Most insects have compound eyes that are composed of independent visual units.
2. Thorax. The thorax consists of three segments, each of which has a pair of legs. Most insects also have two pairs of wings attached to the thorax. In some insects, such as beetles, grasshoppers, and crickets, the outer pair of wings is adapted for protection rather than flight.
3. Abdomen. The abdomen consists of up to 12 segments. Digestion takes place primarily in the stomach, and excretion takes place through organs called Malpighian tubules, which constitute an efficient mechanism for water conservation and were a key adaptation facilitating invasion of the land by arthropods.
Figure 19.24. Modified mouthparts in three kinds of insects.
(a) Mosquito, Culex; (b) alfalfa butterfly, Colias; (c) housefly, Musca domestica.
Although primarily a terrestrial group, insects live in every conceivable habitat on land and in freshwater, and a few have even invaded the sea. About 1 million species have been identified with many others awaiting detection and classification (figure 19.25).
Figure 19.25. Insect diversity.
(a) Some insects have a tough exoskeleton, like this stag beetle (order Coleoptera). (b) Human flea, Pulex irritans (order Siphonaptera). Fleas are flattened laterally, slipping easily through hair. (c) The honeybee, Apis mellifera (order Hymenoptera), is a widely domesticated and efficient pollinator of flowering plants. (d) This pot dragonfly (order Odonata) has a fragile exoskeleton. (e) A true bug, Edessa rufomarginata (order Hemiptera), in Panama. (f) Copulating grasshoppers (order Orthoptera). (g) Luna moth, Actias luna, in Virginia. Luna moths and their relatives are among the most spectacular insects (order Lepidoptera).
Key Learning Outcome 19.10. Arthropods, the most successful animal phylum, have jointed appendages, a rigid exoskeleton, and, in the case of insects, wings.